Lithium-ion secondary batteries are widely used for small-size portable instruments such as cellular phones and laptop computers. A non-graphitizable carbon can be doped and dedoped with a large amount of lithium, which is more than a theoretical capacity of graphite, i.e. 372 mAh/g. Further, the non-graphitizable carbon exhibits excellent input/output performances, cycle durability, and low-temperature property, and thus it is developed and used as a negative electrode material of a lithium-ion secondary battery (Patent literature 1).
In recent years, the lithium-ion secondary batteries have been developed for vehicles, and have been in practical use, in response to increasing concern over environmental issues. In the use thereof for vehicles, the input/output performances, cycle durability, and low-temperature property are required in addition to a large doping and dedoping capacity (i.e. charge-discharge capacity). The non-graphitizable carbon is superior in terms of the above characteristics, and thus suitable therefor.
The non-graphitizable carbon can be obtained by using petroleum pitch or coal pitch, phenol resin, or a plant-derived, organic substance as a carbon source. Among these carbon sources of the non-graphitizable carbon, the plant-derived organic substance can be cultivated, and thus it is a sustainable material. Further, the plant-derived organic substance can be stably supplied at a cheap price. Furthermore, a carbonaceous, negative electrode material obtained by heat-treating the plant-derived organic substance has a large amount of pores, and thus it is expected to have a large charge-discharge capacity (Patent literatures 1 and 2).
However, the carbonaceous material obtained from the plant-derived, organic substance has a large amount of pores, and thus shows a tendency to have a large specific surface area. Further, the plant-derived, organic substance has minerals. However, metallic impurities are harmful to the battery, and therefore it is required to preliminarily remove the minerals. Furthermore, according to the study of the inventors, it is known that pores are occluded by the minerals, and therefore potential charge-discharge capacity cannot be obtained.